1. Field of the Invention
The present invention relates in general to a hybrid drive system for driving a motor vehicle, which is equipped with an engine and a motor/generator. More particularly, the present invention is concerned with such a hybrid vehicle drive system which is compact in construction.
2. Discussion of the Related Art
As a drive system for driving a motor vehicle, there is known a hybrid drive system including (a) an engine operated by combustion of a fuel, (b) a second motor/generator, (c) a planetary gear mechanism having a sun gear and a carrier which are connected to one and the other of the engine and the second motor/generator, and a ring gear connected to an output member, and (d) a first motor/generator connected to the output member of the planetary gear mechanism. An example of this type of hybrid vehicle drive system is disclosed in Publication No. 95208, Mar. 31, 1995, JIDOSHA GIJYUTSU JIREISYU, Intellectual Property Group, Japan Automobile Industry Association, and shown in FIG. 18. This hybrid drive system, which is indicated generally at 10 in FIG. 18, includes: an engine 12; a planetary gear mechanism 20 for mechanically distributing an output of the engine 12 received through a damper 14, to a second motor/generator 16 and an output member (in the form of a gear) 26; and a first motor/generator 22 whose torque is transmitted to the output member 26. The engine 12, damper 14, planetary gear mechanism 20 and second motor/generator 16 are disposed coaxially with each other so as to have a common axis of rotation and are arranged in an axially spaced relationship with each other. The first motor/generator 22 is disposed radially outwardly of and coaxially with the damper 14 and planetary gear mechanism 20.
The planetary gear mechanism 20 includes a sun gear connected to a motor shaft 24 of the second motor/generator 16, a carrier connected to the damper 14, and a ring gear connected to a rotor 22r of the first motor/generator 22. To the output member 26, there is connected the rotor 22r through a connecting member 18. The output member 26 meshes with a large gear 30 provided on an intermediate shaft 28, and a small gear 32 also provided on the intermediate shaft 28 meshes with an input member in the form of a ring gear 345 of a bevel gear type differential gear device 34, so that a drive force is transmitted from the output member 26 to right and left drive wheels (not shown) of the vehicle through the differential gear device 34. The second motor/generator 16 is used as an electric generator which is driven by the engine 12 to generate an electric energy for charging an electric energy storage device such as a battery. The first motor/generator 22 is used an an electric motor, which is operated alone or together with the engine as a drive power source for driving the vehicle. The first motor/generator 22 is required to provide a comparatively large torque, and has a larger size (diameter) than the second motor/generator 16.
In the known hybrid drive system constructed as described above, the planetary gear mechanism 20 is disposed radially inwardly of the rotor 22r of the first motor/generator 22. In this arrangement, the planetary gear mechanism 20 is likely to be adversely influenced by heat generated by the first motor/generator 22.
Also known is a hybrid vehicle drive system including (a) an engine operated by combustion of a fuel and disposed on a first axis, (b) a motor/generator disposed on the first axis, (c) an output member disposed on the first axis, and (d) a differential gear device including a pair of output shafts disposed on a second axis substantially parallel to the first axis, and an input member which is disposed rotatably about the second axis and which is rotated by a drive force received from the output member, so that the drive force is distributed through the pair of output shafts to right and left drive wheels of a motor vehicle. Such a hybrid drive system is disposed such that the first and second axes indicated above are substantially parallel to the transverse direction (width direction) of the motor vehicle, as in a front-engine front-drive vehicle. An example of this type of hybrid drive system is disclosed in JP-A-6-328951, wherein two motor/generators are disposed on the first axis for driving the vehicle and charging the electric energy storage device, respectively, and a gear type speed reducing mechanism is provided between the two motor/generators, so that a drive force is transmitted to the differential gear device through the speed reducing mechanism.
In the hybrid drive system as disclosed in JP-A-6-328951, the input member of the differential gear device is disposed so as to avoid an interference with the motor/generators, with a relatively large distance between the first and second axes. This arrangement suffers from a comparatively large size in the direction perpendicular to the first and second axes, and leads to reduced freedom in installing the hybrid drive system on the vehicle. This freedom is further reduced by the input member of the differential differential gear device, which generally has a relatively large diameter for withstanding a large torque.